This invention relates to dual mode seeker systems employed in air craft and missiles for detecting and guiding the missile or air craft toward a target. More particularly the invention relates to a dual mode seeker system utilizing a microwave antenna that is transparent to infrared radiation allowing the infrared and radar modes of the system to occupy the same central axis of the missile or air craft.
Dual mode seeker systems include an infrared detection system to detect infrared radiation emitted from the target object and a radar system, including a microwave receiver and/or generator to passively detect microwave radiation emitted from the target, or actively detect a target by radiating microwave radiation and detecting waves reflected from the target. In general, infrared detecting systems are particularly well adapted to determine the direction that the target lies from the missile or air craft using the system. While radar systems can also determine the distance that the target lies from the missile or air craft, under particular conditions, such as increased cloud cover, infrared detection systems may become inoperative or decrease in their ability to perform properly, and the seeker system must rely on the microwave system exclusively until the missile becomes close to the target where terminal guidance is provided by the infrared system.
It is therefore highly desirable to employ both detection systems in the missile seeker system. Heretofore dual mode seeker systems have required mounting the two systems on different centerlines displaced from one another. This is so because microwaves of the radar system are known to interfere with infrared detectors, and because radar antennas must be metallic, and therefore are opaque to infrared radiation, they would have to be mounted behind any infrared detector. It can be seen that mounting the radar antenna behind the infrared detector allows microwave radiation to pass through the infrared detector jamming it. If the radar antenna is mounted in front of the infrared detector, infrared radiation is blocked from reaching the detector. Mounting the radar antenna off of the axis of the infrared detector usually requires mounting the antenna outside of the radome and increases the system physical and electronic complexity, weight and cost. It can be appreciated that it is highly desirable to be able to provide for a dual mode missile seeker system that has the radar antenna and infrared detector mounted on the central axis of the radome. Such systems have been devised, such as that disclosed in U.S. Pat. No. 3,165,749 for Microwave Transmissive Optical Radiation Reflectors to S.H. Cushner. While these systems are operationally satisfactory, they are optically complex requiring optical lenses and filters and critical alignment thereof.